runtime/race: add test for issue 7561.
runtime/race: add test for issue 7561.

LGTM=dvyukov
R=rsc, iant, khr, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/76520045

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang